1. Field of Invention
The invention relates to a color conversion method used for preferred color tone processing in color images.
2. Related Art
The quality of color images obtained from currently available input devices (digital cameras, scanners and computer graphics devices) is not perfect. Therefore, these color images need to be further processed to improve the color quality. Two major purposes of doing this are: (1) through color calibrations and corrections, the colors in obtained color images are closer to true colors, which called color reproduction; and (2) the colors are changed into the preferred colors. On the other hand, each output device (printer and display) has various rendering color space or color gamut. Furthermore, each person has preferred colors in the color images, such as skin color, sky color, grass color, etc. While using input and output devices, one thus requires good enough cross media color matching. It is therefore desirable to have a color conversion method to change original image color tones to preferred ones.
The so-called color tone refers to visual perception of colors from the chromaticity distribution of an image in color space. For example, images taken under fluorescent light tends to be bluish. Such an image tone is said to cast a blue (or cold) one. Color tone change is shown corresponding relation between the modified color values (chromaticity) and the original color values. For example, in the RGB (Red, Green, and Blue) color space, if all color values of every pixel in an image are multiplied by a factor of 1.2 the image becomes brighter. In addition, if all red (R) values of every pixel in an image are multiplied by 1.2 in RGB color space, then the image becomes reddish.
Conventional processing methods of preferred color tones include adjusting the tone curve, hue, saturation or brightness. Nevertheless, these methods result in changing chromaticity of all colors in the whole color space.
To improve this situation, another method of chromaticity interpolating in the color space has been developed. This method divides the whole color space into a space comprised of several cubes. There is a 3-direction look-up table (LUT) between the original chromaticity and the preferred chromaticity for the conversion. The conversion method of using this LUT is shown in FIG. 1. First, the color K1 in the original color image (whose color space is TUVO) is converted into another preferred color k1 and K2 to k21. The chromaticity of boundary colors (colors on the lines connecting T, U, V, and O) in the original color image is fixed. Other colors are obtained by interpolation. For example, K4 is moved to k4, K5 is moved to k5, and K3=k3 because the boundary colors are unchanged. However, if the color M and color N are fixed colors, then the colors on the line segment MN become boundary colors. Since the preferred color k21 goes beyond the boundary color (segment MN), the new color k21 will be set as a boundary color k22 and K5 is fixed to k52. In other words, the preferred colors and new colors are calculated by interpolation cannot exceed the boundary colors (MN) defined by connecting the fixed colors. This makes color conversion of preferred color tones unfeasible. Therefore, using this method will result in the following problems:                1. One usually only need to make preferred color conversions to particular colors and its surrounding colors. That is, only the colors in this specific area need to be converted. However, the above-mentioned method makes color conversions for the whole color space. This does not only complicate the preferred color processing and slow down the conversion speed; colors that do not need to be changed are also converted, resulting in uncontrollable preferred color processing.        2. The variation of colors has to be gradient changed around the selecting preferred color and therefore should not be restricted by the limitation of fixed colors. Such limitations may result in unnatural colors.        3. When making preferred color conversions, the relative of each color within an area around the selecting preferred color one should be maintained, allowing for continuous and smooth color variation. However, the previously mentioned method cannot achieve this purpose.        4. As the vector of preferred colors cross with fixed color connection lines (e.g. color K2 becoming color k21), the previous method cannot perform color conversions for the preferred colors. This situation of preferred color conversion, however, is occurred frequently and very important in practice.        